Conventionally, impact protection systems have employed, as the energy absorbing material, elastomeric foams or similar relatively soft, resiliently compressible materials. However, only limited protection is achieved thereby. In some systems, this energy absorbing material is employed in combination with a rigid member the purpose of which is to spread the impact force over a greater area and therefore reduce its effect. However, such systems tend to be inflexible and uncomfortable if in contact with a human body. Most vulnerable areas of the body which require protection, e.g. elbows and knees, undergo significant changes in geometry and thus any attempt to match a rigid load-spreading shape will usually fail. One solution is to introduce articulation into the rigid element but this can compromise performance and increases cost.
More recently, proposals have been made for the use of strain-rate sensitive shear thickening silicone putty materials, sometimes known as silicone dilatants, in or as energy absorbing materials in impact absorption systems. By a strain-rate sensitive shear thickening material or dilatant, we mean a material which viscous flows at low rates of strain deformation but, at an elevated strain rate of deformation undergoes a substantial increase in viscosity with rate of change of deformation. At significantly higher deformation rates, such as those induced by a sudden impact, the material becomes substantially stiff or rigid. For example, U.S. Pat. No. 5,599,290 describes a bone fracture prevention garment which employs, as the dilatant or shear-thickening material, a dispersion of solid particles in a viscously fluid. GB-A-2349798 describes an energy absorbing pad including a putty-like dilatant. However, in both cases, the dilatant has to be contained in an envelope because of its non self-supporting nature. The products therefore tend to have limited flexibility, are prone to damage by puncture, and require relatively complex and expensive manufacturing processes. These products also tend to be heavy due to the relatively high density of the dilatant, which can be above 1000 kg/m3, and suffer from migration of the dilatant within the envelope as the dilatant will exhibit viscous flow at even very low levels of loading.
Other approaches for the utilisation of silicone dilatants have been to combine this material with a resilient carrier such as polyurethane foam.
In our copending International patent publication WO 03/055339 we have described and claimed a self supporting energy absorbing composite comprising:
i) a solid foamed synthetic polymer matrix;
ii) a polymer-based dilatant, different from i), distributed through the matrix and incorporated therein during manufacture of i); and
iii) a fluid distributed through the matrix, the combination of matrix dilatant and fluid being such that the composite is resiliently compressible;
and a self supporting energy absorbing composite comprising:
i) a solid, closed cell foam matrix;
ii) a polymer-based dilatant, different from i), distributed through the matrix; and
iii) a fluid distributed through the matrix, the combination of matrix dilatant and fluid being such that the composite is resiliently compressible.